1. Field of the Invention
The present invention relates to a projection printing apparatus, a projection printing method, a mask pattern for evaluating amplitude aberration, a method of estimating the quantity of amplitude aberration and a filter for eliminating amplitude aberrations, used in LSI (Large Scale Integrated Circuit) manufacturing process.
2. Description of the Background Art
A conventional projection printing apparatus will now be described.
FIG. 22 is a schematic diagram showing a configuration of the conventional projection printing apparatus. In a projection printing apparatus 110, a fly-eye lens 13 is arranged in front of a lamp housing 11 with a mirror 12 interposed therebetween and an aperture 14 is positioned in front of fly-eye lens 13. A blind 16 is arranged in front of aperture 14 with a condenser lens 15 interposed therebetween and is followed by a condenser lens 17, a mirror 18, a condenser lens 19 and a photomask 20 having a desired circuit pattern formed thereon. A wafer 21 is arranged in front of photomask 20 with a projection optical system interposed therebetween.
Projection optical system 106 has a condenser lens 101, a pupil plane or pupilary surface 105 and a condenser lens 102 arranged in front of photomask.
Generally, the limit of resolution R in photolithography employing reduction-type projection printing is represented by the following equation: EQU R=k.sub.1 .multidot..lambda./(NA)
wherein .lambda. represents the wavelength in nm of the light used, NA represents the numerical aperture of the lens used and k.sub.1 represents a constant which depends on resist process.
As can be seen from the above equation, the limit of resolution R can be improved, or a micropattern can be obtained, by reducing the values of k.sub.1 and .lambda. as well as increasing the value of NA, i.e., by reducing a constant which depends on resist process as well as reducing the wavelength of the light used and increasing the value of NA.
However, this method has its limitation, since when the wavelength is reduced and the value of NA is increased, the focal depth .delta. (.delta.=K.sub.2 .multidot..lambda./(NA).sup.2) of the light is reduced and thus resolution is degraded. Furthermore, the reduction of the wavelength of exposure light requires an extensive modification of transfer process. Particularly for a wavelength of exposure light of no more than 170 nm, prolonged ultraviolet irradiation causes point lattice defect, such as color center, in the material of the lens used. Since the generation of color center causes unevenness in transmissivity and refractive index of the lens and thus the lifetime of the lens system is substantially determined, it is increasingly difficult to obtain high resolution with optical systems employing lenses.
There has been an attempt to overcome this difficulty using mirror at a portion of projection optical systems. Such examples are described in detail, for example, in Japanese Patent Laying-Open Nos. 6-181162, 6-181163 and the like.
A projection optical system described in Japanese Patent Laying-Open No. 6-181162 will now be described. FIG. 23 is a schematic view of the projection optical system of the projection printing apparatus disclosed in Japanese Patent Laying-Open No. 6-181162. The projection printing apparatus has a first group of lenses 201 and 202 having a positive refractive index, a light beam splitter 203, a second group 204 of lenses having a negative refractive index, a concave mirror 205 and a third group 206 of lenses having a positive refractive index in front of a photomask 20.
In the projection printing apparatus, the diffracted light from photomask 20 is transmitted through the first group of lenses 201 and 202, light beam splitter 203 and the second group 204 of lenses and reflected from convex mirror 205. The diffracted light reflected from convex mirror 205 is again transmitted through the second group 204 of lenses, reflected from light beam splitter 203, transmitted through the third group 206 of lenses and thus forms an image on the exposed surface of a wafer 21.
While the aforementioned projection printing apparatus uses mirror at a portion of the projection optical system, it still uses many lenses 201, 202, 204 and 206. Thus, the projection printing apparatus cannot completely resolve unevenness in transmissivity associated with the degradation of the material of lens caused by the reduction of the wavelength of exposure light. Furthermore, when the wavelength of exposure light is no more than 170 nm, color center is also caused in a halfmirror, such as light beam splitter 203, as well as in the lens mentioned above, and thus the degradation of its material associated with the generation of color center will causes an uneven distribution of transmissivity (an unevenness in its transmissivity).
Meanwhile, an example wherein lenses are completely removed is found as an optical system described in Japanese Patent Laying-Open No. 8-54738. FIG. 24 is a schematic view of a configuration of the projection optical system of the projection printing apparatus disclosed in Japanese Patent Laying-Open No. 8-54738. The projection printing apparatus has an aperture 301, a convex mirror 302 and a concave mirror 303.
In this projection printing apparatus, the diffracted light from photomask 20 is transmitted through aperture 301 and then reflected from convex mirror 302 and then by concave mirror 303 and then forms an image on exposed substrate 21.
Since the projection printing apparatus does not use a lens, uneven transmissivity due to degradation of lens material is not caused. However, the light incident on convex mirror 302 from exactly above in the figure, such as zero-order diffracted light, is directly reflected and thus cannot illuminate exposed substrate 21, i.e., the so-called steric hindrance is disadvantageously caused.
Even if zero-order diffracted light beam should illuminate exposed substrate 21, the diffracted light beams on the right and left sides of the zero-order diffracted light beam behave differently and thus satisfactory image characteristics cannot be obtained. For example, assuming that zero-order diffracted light beam is reflected from convex mirror 302 and then by the right portion of concave mirror 303 in the figure and illuminates exposed substrate 21, when positive and negative, first-order diffracted light beams are reflected at the right and left portions of concave mirror 303 in the figure, respectively, illuminate exposed substrate 21, the angle of incidence of the positive and negative, first-order diffracted light beams are different from each other with respect to the zero-order diffracted light beam and thus satisfactory image characteristic cannot be obtained.
Furthermore, due to the behaviors of the diffracted light beams mentioned above, the conditions for imaging the longitudinal pattern of photomask 20 on exposed substrate 21 are different from those for imaging the lateral pattern of photomask 20 on exposed substrate 21 and thus satisfactory image characteristics of the longitudinal and lateral patterns cannot be obtained.
Furthermore, it is generally difficult to eliminate wavefront aberration in mirror systems and thus a portion having less wavefront aberration should be selected for use.
Typical aberrations are spherical aberration, astigmatism aberration, field curvature, distortion aberration and coma aberration. It is known that these aberrations can be expressed, as illustrated in FIGS. 25A-25E, respectively, by conversion into wavefront aberration on pupil surface, FIGS. 25A, 25B, 25C, 25D and 25E illustrating spherical aberration, astigmatism aberration, field curvature, distortion aberration and coma aberration, respectively. In the figures, .phi. represents the quantity in shift of a wavefront at a pupil plane; .rho. the radius on the pupil plane (.eta..xi. plane); .theta. the angle of rotation with respect to the .eta. axis; y.sub.0 coordinates on a wafer surface; and B to F constants. The details of these aberrations are described, for example, in "Principle of Optics I-III" (published by Tokai University Press).